Rosetta Picks Asteroid Targets

Earlier this month, Europe’s Rosetta cometary probe was successfully launched into an orbit around the Sun, which will allow it to reach the comet 67P/Churyumov-Gerasimenko in 2014 after three flybys of the Earth and one of Mars. During this 10-year journey, the probe will pass close to two asteroids.

First images of Annefrank asteroid from StardustCredit: NASA/JPL, U. Wash

The Rosetta Science Working Team has made the final selection of the asteroids that Rosetta will observe at close quarters during its journey to Comet 67P/Churyumov-Gerasimenko. Steins and Lutetia lie in the asteroid belt between the orbits of Mars and Jupiter.

Rosetta’s scientific goals always included the possibility of studying one or more asteroids from close range. However, only after Rosetta’s launch and its insertion into interplanetary orbit could the ESA mission managers assess how much fuel was actually available for fly-bys. Information from the European Space Operations Centre (ESOC) in Germany enabled Rosetta’s Science Working Team to select a pair of asteroids of high scientific interest, well within the fuel budget.

Illustration of Rosetta sitting on comet surface Credit: ESA

The selection of these two excellent targets was made possible by the high accuracy with which the Ariane 5 delivered the spacecraft into its orbit. This of course leaves sufficient fuel for the core part of the mission, orbiting Comet 67P/Churyumov-Gerasimenko for 17 months when Rosetta reaches its target in 2014. The comet will then still be far from the Sun and its nucleus should be dormant.

Study of its orbit shows that it was captured quite recently, after too close encounters with Jupiter in 1840 and 1959. The comet is now revolving around the Sun every 6.6 years on an elliptical orbit with a low inclination compared to that of the Earth. Being a relatively ‘fresh’ comet in the inner Solar System, 67P/Churyumov-Gerasimenko is a promising target for the study of primitive Solar System material.

The first planetary encounter will be in March 2005, as Rosetta flies by the Earth for the first time. The gravity assist will boost Rosetta into an orbit that will take it to Mars two years later.

Asteroids are primitive building blocks of the Solar System, left over from the time of its formation about 4600 million years ago. Only a few asteroids have so far been observed from nearby. They are very different in shape and size, ranging from a few kilometers to over 100 kilometers across, and in their composition.

The rocks inside a crater on the Asteroid Eros. Numerous small impacts on the asteroid show brown boulders visible interior to the less exposed (white) lip of the crater. False-color for emphasis. Credit: NEAR Project, JHU APL, NASA

The targets selected for Rosetta, Steins and Lutetia, have rather different properties. Steins is relatively small, with a diameter of a few kilometers, and will be visited by Rosetta on 5 September 2008 at a distance of just over 1700 kilometers. This encounter will take place at a relatively low speed of about 9 kilometers per second during Rosetta’s first excursion into the asteroid belt.

Lutetia is a much bigger object, about 100 kilometers in diameter. Rosetta will pass within about 3000 kilometers on 10 July 2010 at a speed of 15 kilometers per second. This will be during Rosetta’s second passage through the asteroid belt.

Rosetta will obtain spectacular images as it flies by these primordial rocks. Its onboard instruments will provide information on the mass and density of the asteroids, thus telling us more about their composition, and will also measure their subsurface temperature and look for gas and dust around them.

The approximately 4 kilometer cometary nucleus to be landed upon by the Rosetta and its harpooning landing hooksCredit: Hubble

Past missions that have flown by a comet have been: NASA’s ICE mission in 1985, the two Russian Vega spacecraft and the two Japanese spacecraft Suisei and Sakigake that were part of the armada that visited Comet Halley in 1986; NASA’s Deep Space 1 flew by Comet Borelly in 2001 and NASA’s Stardust, which flew by Comet Wild 2 earlier in January and has captured samples of the comet’s coma to be returned in 2006. Unfortunately NASA’s Contour launched in summer 2002 failed when it was inserted into its interplanetary trajectory. Later this year will also hail the launch of Deep Impact, a spacecraft that will shoot a massive block of copper into a comet’s nucleus.

On Valentine’s Day, 2001, the Near-Shoemaker spacecraft successfully landed on the asteroid, Eros. Its remarkable journey–to soft-land on a peanut shaped asteroid - about 176 million kilometers (109 million miles) from Earth, prompted Andrew Cheng, NEAR Project Scientist, to note: "On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life."

"Comets and asteroids are the building blocks of our Earth and the other planets in the Solar System. Rosetta will conduct the most thorough analysis so far of three of these objects," said Prof. David Southwood, Director of ESA’s Science Programme. "Rosetta will face lots of challenges during its 12-year journey, but the scientific insights that we will gain into the origin of the Solar System and, possibly, of life are more than rewarding."